Conventional color photography depends on the formation of dyes and uses subtractive primaries to form the desired colors. One of the problems encountered with the dyes which form the images is their tendency to degrade when exposed to light. This is of particular importance with respect to photographic elements intended for direct viewing. Direct view elements include reflective prints and color transmission elements such as motion picture prints and projection slides. Such elements receive substantial exposure to light while being viewed. This not only causes the dye images to fade, but when the dyes of different colors fade at different rates, the image changes color and the neutral areas become undesirably colored. Since the advent of color photography there have been ongoing efforts to improve the dye stability. Nevertheless, there is still a need for further improvement in the dye light stability of photographic elements intended for direct viewing. Efforts have been ongoing to provide stabilized dyes which will exhibit improved light stability. Some of these efforts are described as follows.
EP0 704 758 of Kawagishi et al. teaches certain 1H-pyrazolo[5,1-c]-1,2,4-triazole compounds which are said to be useful for improving color reproduction, color developability, and magenta dye light stability. The coupler contains at the 3-position an aminoalkylene group bonded to a hetero substituted phenyl group by a --C(O)--, --SO.sub.2 --, --SO.sub.2 NR--, --C(O)NR--, or --COOR-- group. Neither the carbon of the aminoalkylene group alpha or beta to the azole ring to which it is bonded is required to be further substituted. Among the specific couplers contemplated are those having, for example, .alpha.-methyl, .alpha.-isopropyl, .alpha.-ethyl, .alpha.-dodecyl, .alpha.-phenyl and .alpha.,.alpha.-dimethyl links to the azole ring. In almost every example, the .alpha. carbon is linked directly to a heterogroup of the substituent. When there is present a .beta.-carbon, it is never further substituted.
EP 602 748 of Tang et al. suggests generic substituent groups for pyrazolotriazole couplers which enhance the combination of color reproduction, dye light stability and coupler reactivity. It is stated that substitution at the alpha carbon is desired, but there is no specific suggestion that further substitution at the beta carbon is advantageous.
U.S. Pat. No. 5,470,697 of Kita et al. suggests pyrazolotriazole couplers having a fully substituted carbon in the alpha position with an oxygen linked moiety in two of the branches for purposes of improved sensitivity and reduced development pH sensitivity No data on dye stability is given and there is no recognition of the advantage of any particular substituent combination for purposes of improving dye light stability.
U.S. Pat. No. 5,032,497 of Nakayama et al. suggests a certain pyrazolotriazole coupler having a tertiary alkyl group in the 3-position and a primary alkyl group in the 6-position for the purpose of improving the absorption spectra of the magenta dye formed upon development and improving the resistance of the dye to formaldehyde and light.
In addition to the desired light stability of the resulting magenta dye, it is also important that the magenta coupler be manufacturable in a commercial sense. Purification is an integral part of the synthesis of compounds. There are many methods of purification known in the art such as crystallization, distillation, chromatography, and dialysis. These methods find wide application in academia and in industry. Many of these methods, however, are limited in that they can handle only relatively small amounts of material. For example, chromatography is an extremely efficient method of purification of organic compounds but when applied to quantities over 25 grams, the quantities of absorbent, the volumes of solvents needed, and the physical operations required (such as evaporation of elutant solvents), make the operation impractical for large quantities from a cost point of view.
Solids have another advantage during manufacture. Compounds need to be transferred between reaction vessels and between areas of manufacture and areas of use (such as dispersion making in the case of photographic couplers). The transfer of liquids and oils, especially high molecular weight materials which tend to be very viscous, can be problematic. The transfer of solids is easier.
For the manufacture of large amounts of material, it is highly desirable to use purification techniques that are cost effective for large amounts of material. For the manufacture of photographic couplers, hundreds of kilograms a year or more would be needed and thus a purification method such as chromatography as described above would be impractical. The two practical methods of purification in the manufacture of large amounts of material are distillation and recrystallization. In order to use distillation, the material must boil at temperatures less than 140.degree. C. at reduced pressure. Photographic couplers are typically too high boiling to distill, and therefore recrystallization is the only practical way to purify large quantities of such materials.
In order to purify materials by recrystallization, it is necessary that the materials have a well defined crystal structure. Materials having this well defined crystal structure are characterized by a sharp melting point when pure; the melting point range is typically less than 2.degree. C. A discussion of recrystallization can be found in Purification of Laboratory Chemicals by D. D. Perrin and W. L. F. Armarego, Pergamon Press, N.Y. 1988.
Thus it is a problem to be solved to provide a magenta coupler that not only forms a dye having advantageous resistance to light degradation but, from the practical standpoint, one that is crystallizable in order to be capable of industrial exploitation.